This adult female was found inside a cockle shell. Her dark eggs that are clearly visible through her soft and translucent carapace. In the bottom left image the crab is upside-down, with eggs nearly bursting out from the sides of her abdomen. She is dorsal side up in the top left and right images, but barely able to touch her walking legs down due to her eggs. Photos by Jenn Burt.

Identification​The grooved mussel crab goes through two very different adult life stages. In the first, a planktonic stage, the crab is hard-shelled and has dense fringes of setae lining its legs, which allow it to swim. It also has a thickened "brow" spot at the front of its carapace. At this point the crab is mainly opaque white and tan, with some reddish spots on the carapace. During the second stage, which only adult females transition into, the carapace is soft, translucent, and has grooves extending back from each eye. While adult males only reach about 7 mm across, adult females can be more than triple that size at 22 mm.

​Intriguing InfoUpon reaching sexual maturity, both males and females emerge from their bivalve hosts and transform into the hard-shelled swimming phase, at which point they mate. Once this stage is complete, females return to hosts to brood their eggs. Whether any males return to hosts as well is unclear, though it appears as though most males die after mating. Mating takes place during spring and/or summer, and females brood eggs over winter.

The grooved mussel crab is considered to have a parasitic relationship with its host rather than commensalistic or mutualistic due to its tendency to damage its host's gills. It also steals food from its host by feeding on the mucous that the bivalve produces during filter feeding. This mucous is found in the host's gills.